Description

The
tzset()
function initializes the tzname variable from the
TZ
environment variable.
This function is automatically called by the
other time conversion functions that depend on the timezone.
In a System-V-like environment, it will also set the variables timezone
(seconds West of UTC) and daylight (to 0 if this timezone does not
have any daylight saving time rules, or to nonzero if there is a time during
the year when daylight saving time applies).

If the
TZ
variable does not appear in the environment, the system timezone is used.
The system timezone is configured by copying, or linking, a file in the
tzfile(5) format to
/etc/localtime.
A timezone database of these files may be located in the system
timezone directory (see the FILES section below).

If the
TZ
variable does appear in the environment, but its value is empty,
or its value cannot be interpreted using any of the formats specified
below, then Coordinated Universal Time (UTC) is used.

The value of
TZ
can be one of two formats.
The first format is a string of characters that directly represent the
timezone to be used:

std offset[dst[offset][,start[/time],end[/time]]]

There are no spaces in the specification.
The std string specifies the name of the timezone and must be
three or more alphabetic characters.
The offset string immediately
follows std and specifies the time value to be added to the local
time to get Coordinated Universal Time (UTC).
The offset is positive
if the local timezone is west of the Prime Meridian and negative if it is
east.
The hour must be between 0 and 24, and the minutes and seconds 00 and 59:

[+|-]hh[:mm[:ss]]

The dst string and offset specify the name and offset for the
corresponding daylight saving timezone.
If the offset is omitted,
it defaults to one hour ahead of standard time.

The start field specifies when daylight saving time goes into
effect and the end field specifies when the change is made back to
standard time.
These fields may have the following formats:

Jn

This specifies the Julian day with n between 1 and 365.
Leap days are not counted.
In this format, February 29 can't be represented;
February 28 is day 59, and March 1 is always day 60.

n

This specifies the zero-based Julian day with n between 0 and 365.
February 29 is counted in leap years.

Mm.w.d

This specifies day d (0 <= d <= 6) of week w
(1 <= w <= 5) of month m (1 <= m <= 12).
Week 1 is
the first week in which day d occurs and week 5 is the last week
in which day d occurs.
Day 0 is a Sunday.

The time fields specify when, in the local time currently in effect,
the change to the other time occurs.
If omitted, the default is 02:00:00.

Here is an example for New Zealand,
where the standard time (NZST) is 12 hours ahead of UTC,
and daylight saving time (NZDT), 13 hours ahead of UTC,
runs from the first Sunday in October to the third Sunday in March,
and the changeovers happen at the default time of 02:00:00:

TZ="NZST-12:00:00NZDT-13:00:00,M10.1.0,M3.3.0"

The second format specifies that the timezone information should be read
from a file:

:[filespec]

If the file specification filespec is omitted, or its value cannot
be interpreted, then Coordinated Universal Time (UTC) is used.
If filespec is given, it specifies another
tzfile(5)-format
file to read the timezone information from.
If filespec does not begin with a aq/aq, the file specification is
relative to the system timezone directory. If the colon is omitted each
of the above TZ formats will be tried.

Here's an example, once more for New Zealand:

TZ=":Pacific/Auckland"

Environment

TZ

If this variable is set its value takes precedence over the system
configured timezone.

TZDIR

If this variable is set its value takes precedence over the system
configured timezone database directory path.

Files

/etc/localtime

The system timezone file.

/usr/share/zoneinfo/

The system timezone database directory.

/usr/share/zoneinfo/posixrules

When a TZ string includes a dst timezone without anything following it,
then this file is used for the start/end rules.
It is in the
tzfile(5) format.
By default, the zoneinfo Makefile hard links it to the
America/New_York tzfile.

Above are the current standard file locations, but they are
configurable when glibc is compiled.

Conforming To

SVr4, POSIX.1-2001, 4.3BSD.

Notes

Note that the variable daylight does not indicate that daylight
saving time applies right now.
It used to give the number of some
algorithm (see the variable tz_dsttime in
gettimeofday(2)).
It has been obsolete for many years but is required by SUSv2.

4.3BSD had a function
char *timezone(zone, dst)
that returned the
name of the timezone corresponding to its first argument (minutes
West of UTC).
If the second argument was 0, the standard name was used,
otherwise the daylight saving time version.

See Also

Colophon

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License & Copyright

Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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References consulted:
Linux libc source code
Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
386BSD man pages
Modified Sun Jul 25 11:01:58 1993 by Rik Faith (faith@cs.unc.edu)
Modified 2001-11-13, aeb
Modified 2004-12-01 mtk and Martin Schulze